Electroluminescent element, hereinafter also referred to as EL element, is superior in the visibility since it emits light itself, and the element including the driving circuit for it can be made light because the element can be driven by a low voltage of from several to several tens volt. Therefore, the organic EL element is expected to be applied for thin type displays, illuminators and backlights.
It is also a feature of the organic EL element that it has abundant color variation. Moreover, it is also a feature that various colors can be obtained by mixing plural colors of the emitted light.
Among the colors of the emitted light, need for white color light is especially high, such the light can be also applied for the backlight. Moreover, the white light can be separated into blue, green and red pixels.
For emitting such the white light, the following two methods can be applied.
1. To dope plural light emitting compounds into a light emission layer
2. To combine plural colors of light each emitted from plural light emission layers
For instance, when the above method 1 using a vacuum vapor deposition method is applied for attaining such the white color by mixing by three colors of blue (B), green (G) and red (R), a vapor deposition of four elements of B, G, R and a host compound should be carried out, which is difficulty controlled.
In another way, a method can be applied, in which a solution formed by dissolving or dispersing the B, G, R and host compounds is coated. However, it is a problem that the coated type EL element is inferior in the durability to the vapor deposited type element.
On the other hand, a method of the above 2 by combining plural light emission layers is proposed. Such the method is easier than the method of 1 when the vapor deposition is applied.
As such the white light emission EL element, an element is proposed in which a blue light emitting layer as a short wavelength light emission layer and a red light emission layer as a long wavelength light emission layer are laminated so as to emit white light by mixing light each emitted from these two light emission layers, for example, cf. Patent Document 1.
However, in the element having the two laminated light emission layers different from each other in the color of the emitting light (different in the peak wavelength of emitted light), the light emitting center is moved by variation in the layer quality or transportability of the positive hole and the electron accompanied with the variation in the driving time or variation in the light emitting time and the applied voltage so that the chromaticity tends to be varied.
Such the problem is actualized when white light is obtained by mixing light emitted from the two light emission layers since white light is sensitive for variation in the chromaticity.
An EL element emitting mixed light composed of lights each different in the peak wavelength emitted from each of plural light emission layers is disclosed, in which three or more light emission layers each emitting light different in the peak wavelength are alternatively laminated, as a method for inhibiting the chromaticity variation caused by the variation in the driving time and the voltage as low as possible, for example, cf. Patent Document 2.
Moreover, a method for designing the laminated structure having two or more layers utilizing the thickness of the light emission layer and the ratio of an organic host material to a fluorescent material as parameters of the light emission efficiency, for example, cf. Patent Document 3.
The above means are effective for inhibiting chromaticity deviation by alternatively laminating the layers even when the balance of injection of the carriers is lost in some degree. It is found, however, that the light emitting efficiency is low, energy transfer between the layers is caused and deviation in the whiteness is observed so that such the elements are insufficient in the white light emission.
As an example of attaining white light by the combination of plural light emission layers, an element having two light emitting layers each different in the emitting light color and an intermediate layer arranged between the light emission layers is disclosed, for example, cf. Non-patent Document 1.
The above technique, however, has the following problems:
1. Color of emitted light is deviated depending on the applied voltage (electric current).
2. The efficiency is not attained to the theoretical limit.
3. The process is made complex by the formation of the intermediate layer.
The above problems remain as the problems to be solved.
Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 7-142169
Patent Document 2: JP-A No. 2003-187977
Patent Document 3: JP-A No. 2004-63349
Non-patent Document 1: Applied Physics Letters, Vol. 83, 2459 (2003) and Advanced Materials, Vol. 14, No. 2, 147 (2002)